The present invention relates to wavelength lockers for tunable lasers, and more particularly to a wavelength locker including a planar waveguide.
Wavelength lockers are required for wavelength division multiplexing (WDM) systems with narrow channel spacings. Wavelength lockers are particularly important for tunable lasers, particularly when the tunable lasers need to lock to multiple wavelength channels. Ideally, a wavelength locker is capable of locking multiple channels. In addition to tunable lasers, there are many other applications that require an optical signal to have a stable wavelength within predetermined limits. Transmission of multiple signals having different stable wavelengths allows the transmission of different channels.
Commercially available devices that perform the function of wavelength locking are costly, complicated, and need very precise alignment. Some conventional wavelength lockers are based on fiber gratings or Fabry-Perot etalons. Since the fiber gratings are narrow band and generally made serially in a fiber, they can lock onto only one or a few channels. For example, a tunable laser with 12 nm tuning range can access 32 channels that are spaced 50 GHz apart. A fiber grating that locks onto all of the channels needs to have 32 separate fiber gratings, which is difficult and costly to implement.
Alternatively, a bulk Fabry-Perot etalon having a precise thickness and resonances at multiple wavelengths can be employed. For example, U.S. Pat. No. 5,798,859, which is hereby incorporated by reference, discloses a wavelength locker for a tunable laser module that employs a bulk Fabry-Perot etalon. The wavelength locker is compact and can be integrated into the laser package. A temperature insensitive etalon is fabricated by using a combination of materials with low or zero coefficients of thermal expansion. This obviates the need for additional temperature stabilization but requires extremely precise alignment to set the wavelength of the etalon. Alternatively, if a solid material is used during fabrication, the etalon will have some temperature sensitivity and must be temperature stabilized. The precise wavelength can be adjusted by varying the operating temperature set point.
An exemplary wavelength locker implementation includes a Fabry-Perot etalon for a 12 nm tunable laser with 50 GHz channel spacing. For both temperature insensitive and sensitive packages, the exact channel spacing is fine-tuned by adjusting the tilt of the etalon relative to the incident light. For example, a 2 mm thick piece of quartz has a channel spacing of 51 GHz. The channel spacing can be adjusted to 50 GHz by tilting the etalon to an angle of 13.26 degrees. To keep the channels aligned within {fraction (1/10)} of the channel spacing over 12 nm, the spacing must be held within {fraction (1/10)}xc3x97{fraction (1/32)} of the channel spacing (or about 0.16 GHz). This requires a tilt alignment accuracy within 0.1 degrees. For the temperature insensitive package, the absolute wavelength is also adjusted by tilting the elaton. For {fraction (1/10)} channel spacing, the tilt alignment must be within {fraction (1/200)} of a degree accuracy. The temperature sensitive package can be set by controlling the operating temperature. When quartz is used, the operating temperature must be controlled with an accuracy of 4xc2x0 C. In either case, the exact alignment of the etalon has proven to be difficult. Even when the temperature sensitive package is employed, obtaining alignment within 0.1 degrees can be difficult to achieve.
A tunable laser according to the invention includes a laser operating at a first wavelength. A wavelength locker includes a planar waveguide that is coupled to the laser. The wavelength locker tunes the laser to a first wavelength value.
In other features of the invention, the waveguide wavelength locker includes a detector. The wavelength locker and the detector generate an error signal based on a difference between the first wavelength value and a desired wavelength value. A controller is connected to the waveguide wavelength locker and the laser. The controller generates a laser control signal based on the error signal. The laser control signal adjusts the first wavelength value to the desired wavelength value.
In other features of the invention, the waveguide wavelength locker includes a glass waveguide with a first strong grating that is spaced from a second strong grating to form a Fabry-Perot cavity.
In yet other features of the invention, the waveguide wavelength locker includes a passive waveguide that is connected to a Mach-Zehnder interferometer having first and second arms with unequal lengths. The Mach-Zehnder interferometer is connected to a first detector. The first detector generates an alternating signal with peaks that are spaced as a function of wavelength. A grating is connected to a second detector. The second detector generates a reference signal having a peak at a fixed wavelength. The waveguide wavelength locker further includes a passive waveguide that is connected to a third detector. The third detector generates a normalization signal. The controller receives the alternating signal, the reference signal and the normalization signal and generates the control signal therefrom.
In other features, the laser is mounted on a first temperature controlled package and the waveguide wavelength locker is mounted on the first temperature controlled package.
In still other features, the waveguide wavelength locker includes first, second and third Mach-Zehnder interferometers with different asymmetries. The first, second and third Mach-Zehnder interferometers are connected to first, second and third detectors. A passive coupler is connected to a fourth detector. The first, second, third and fourth detectors are connected to the controller. The controller uses outputs of the first, second, third and fourth detectors to access a lookup table for faster wavelength measurement.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.